1. Field of the Invention
The present invention relates to a quadruple-tandem type image forming apparatus employed for an electrophotogaphic process copying machine, a printer or the like.
2. Related Art Statement
FIG. 1 is a sectional view of a general quadruple-tandem type image forming apparatus, and printing operation of a color image will be explained with reference to FIG. 1. Here, as photosensitive body drums 103axcx9c103d, OPC (Organic Photo Conductor) is employed.
The image forming steps in an image forming unit al will be explained. First, the surface of a photosensitive body drum 103a is uniformly charged in minus (xe2x88x92) by a scorothoron charger 105a. An exposure device not shown is disposed downstream of the scorothoron charger 105a, and exposure in conformity with image information is applied to the charged photosensitive body 103a to thereby form an electrostatic latent image. A 2-component developing unit 109a for encasing a yellow developer is disposed downstream of the exposure device, an electrostatic latent image is reversal-developed by a yellow toner, and a toner image is formed on the photosensitive body 103a. 
On the other hand, a sheet of paper P as a transfer material are supplied from a paper cassette not shown and conveyed, and paper P is then transported onto a conveying belt 111 by an aligning roller 114 adjusting to a timing at which a toner image is formed on the photosensitive body 103a. A bias voltage (+) reversed in polarity to a charge polarity of a toner is applied by a DC power supply 127a to a transfer roller 123a, and a toner image on the photosensitive body 103a is transferred onto paper P by a transfer electric field formed between the photosensitive body 103a and the transfer roller 123a. 
Here, a partial toner (a residual transfer toner) remained on the photosensitive body 103a without being transferred completely on paper P is cleaned by a photosensitive body cleaner 120a, and is sent as a waste toner to a waste toner box not shown. The cleaned surface of the photosensitive body 103a is exposed by an eliminator 121a and thereby eliminated, after which the steps of charge, exposure and development are repeated.
Also in image forming units b1, c1 and d1, a toner image is formed by the process similar to that mentioned above adjusting to a timing at which the toner image is formed in the image forming unit a1, and toner images of magenta, cyan, and black formed on the photosensitive bodies 103b, 103c, and 103d of the image forming units b1, c1 and d1 are also sequentially transferred onto paper P conveyed by the conveying belt 111. Generally, there is no uniformity in relation between an charge potential of a photosensitive body of each image forming unit, an charge quantity of toner, and a development potential.
A fixing unit not shown for fixing a toner on paper P is disposed downstream of the conveying belt 111, and paper P is caused to pass through the fixing unit to obtain a fixed image.
As described above, in the transfer step, a toner on the photosensitive body is transferred to a transfer material by a transfer electric field generated between the photosensitive body and the transfer roller, but there poses a problem in that where the transfer electric field is large, there occurs a so-called reversal transfer phenomenon in which the toner once transferred to the transfer material is returned to the photosensitive body again.
As the procedure for suppressing such a reversal transfer phenomenon as described, there has been proposed a procedure described in Japanese Patent Application Laid-Open No. 209232/2001 in which the charge quantity of toner downstream in a moving direction is set to be lower than the charge quantity of toner upstream, and a transfer bias voltage is set to be lower closer to the downstream. However, even if the transfer bias voltage is the same, a tendency of the reversal transfer is changed by a surface potential of the photosensitive body, as illustrated in the embodiment described later, from which it is said that generally, a grade need not be provided relative to the charge quantity of toner or the transfer bias voltage, and further, there is a disadvantage that a variation in reproducibility or gradient of dots of a color toner occurs by the mere provision of a grade relative to the charge quantity of toner.
Further, there has been also proposed a procedure described in Japanese Patent Application Laid-Open No. 209232/2001 in which a development potential in toner image forming means downstream is set to be lower than that upstream, and |VLxe2x88x92Vbias| is made smaller closer to downstream to thereby make electrostatic attracting force exerted between a toner of reversal polarity and the photosensitive body small. However, also in this respect, as illustrated in the embodiment described later, a development contrast or a development quantity of toner is not directly related to the reversal transfer, and in addition, there is a problem also that a variation in reproducibility or gradient of dots of a color toner occurs by the mere provision of a grade relative to the development potential.
With respect to such a reversal transfer phenomenon as described, the inventor thought that it resulted from a Pachen-discharge generated in the vicinity of a transfer region because of the fact that where a difference between a potential (normally, an earth potential) at the back of the photosensitive body or an charge potential of the surface of the photosensitive body and the effective value of a transfer bias voltage is large, the above-described phenomenon occurs often, and that an charge quantity of toner on the transfer material after having passed through the transfer region increases than that prior to the passage whereas an charge quantity of the reversal transfer toner considerably lowers (becoming+polarity).
Therefore, preferably, the transfer electric field is controlled so as not to generate the Pachen-discharge in order to suppress the reversal transfer. As means for controlling the transfer electric field, there have been generally known a method for controlling the transfer bias voltage itself, and a method for, before transferring a toner on the photosensitive body to a transfer material, exposing it to lower a potential of the surface of the photosensitive body (elimination before transfer) to lower a transfer electric field. However, by merely setting the transfer bias voltage to be lower, the reversal transfer can be reduced but the residual transfer increases, and the transfer efficiency is lowered, posting a problem in reproducibility of an image. Further, in case of elimination before transfer, the reversal transfer can be reduced, but there poses a problem that when a charge on the photosensitive body disappears, Coulomb repulsion force caused by toners is affected to increase dusts on the image.
Setting an charge potential of the photosensitive body lower is also one means for suppressing the reversal transfer, but there is a problem that when the charge potential of the photosensitive body is set to be lower, an image concentration lowers or a reproducibility or gradient of dots lowers also according to the charge quantity of toner or development bias voltage.
In a so-called photosensitive body cleanerless system in which a cleaner for cleaning a toner on the photosensitive body is not disposed but cleaning is carried out simultaneously with development by a developing unit, a toner remained on the photosensitive body without being transferred to a transfer material (a residual transfer toner) can be recovered into the developing unit, thus providing the merit that a waste toner can be reduced, and the service life of the photosensitive body extends, whereas there is a great problem that when the reversal transfer phenomenon occurs simultaneously in a plurality of colors of toners, toners are mixed in color within the developing unit, and such a method as described is not practically used in the quadruple-tandem type image forming apparatus.
The present invention has been accomplished in view of the problem noted above with respect to prior art, and an object of the invention is to provide a quadruple-tandem type image forming apparatus capable of securing the reproducibility or gradient of image dots of colors and image concentration simultaneously with the reduction in reversal transfer.
It is a further object of the invention to provide a quadruple-tandem type image forming apparatus which employs a photosensitive body cleanerless process in consideration of the environment, capable of preventing mixing of colors of toners by reducing a reversal transfer, reducing a waste toner and extending the service life of the photosensitive body while securing the reproducibility or gradient of image dots of colors and image concentration.
For achieving the aforementioned objects, according to the present invention, there is provided a quadruple-tandem type image forming apparatus, disposing four units of an image forming unit comprising: a photosensitive body, an charger for charging the photosensitive body to a prescribed potential, an exposure device for forming an electrostatic latent image on the photosensitive body, and a developing unit for forming a toner image on the photosensitive body, the toner images being transferred to a transfer material sequentially, characterized in that supposing that surface potentials of the photosensitive body in each image forming unit are V0a, V0b, V0c, V0d from upstream toward downstream in a transfer material moving direction, development bias voltages in each image forming unit are VBa, VBb, VBc, VBd, and charge quantities of toners in the developing units in each image forming unit are qa, qb, qc, qd, the following relation is fulfilled:
V0axe2x89xa7V0bxe2x89xa7V0cxe2x89xa7V0d
VBaxe2x89xa7VBbxe2x89xa7VBcxe2x89xa7VBd
qaxe2x89xa7qbxe2x89xa7qcxe2x89xa7qd
(wherein V0a greater than V0d, VBa greater than VBd, qa greater than qd)
According to the present invention, there is further provided a quadruple-tandem type image forming apparatus, disposing four units of an image forming unit, comprising: a photosensitive body, an charger for charging the photosensitive body to a prescribed potential, an exposure device for forming an electrostatic latent image on the photosensitive body, and a developing unit for forming a toner image on the photosensitive body, employing a cleanerless system without disposing a cleaner for cleaning a toner on the photosensitive body and carrying out cleaning simultaneously with the development by the developing unit, the toner images being transferred to a transfer material sequentially, characterized in that supposing that surface potentials of the photosensitive body in each image forming unit are V0a, V0b, V0c, V0d from upstream toward downstream in a transfer material moving direction, development bias voltages in each image forming unit are VBa, VBb, VBc, VBd, and charge quantities of toners in the developing units in each image forming unit are qa, qb, qc, qd, the following relation is fulfilled:
V0axe2x89xa7V0bxe2x89xa7V0cxe2x89xa7V0d
VBaxe2x89xa7VBbxe2x89xa7VBcxe2x89xa7VBd
qaxe2x89xa7qbxe2x89xa7qcxe2x89xa7qd
(wherein V0a greater than V0d, VBa greater than VBd, qa greater than qd).